1. Field of the Invention
The present invention relates to a method for accessing a cell, and more particularly, to a method for acquiring an initial synchronization using two pilot channels in a CDMA (Code Division Multiple Access) communication system of an asynchronous or quasi-synchronous mode.
2. Description of the Related Art
In general, in the microcell environment having a plurality of base stations, since expensive equipment such as GPS (Global Positioning System) is required in each base station, the cost can be quite high. Also, in the case where the base station is set up in an underground space and room, the use of the GPS equipment has many technical difficulties.
Further, in the CDMA communication system, which is employed according to the timing information outside a restricted area, a system capable of being independently operated may be required because there is no method for coping with troubles in the GPS and with an intentional information intercept caused by the system operator. For example, the CDMA communication system, which can be operated without the use of the GPS, has been developed by some companies in Europe and Japan. However, there still remains many unsolved problems. FIG. 1 is a schematic view illustrating a method of discriminating a base station of the reception frequency using the conventional time offset.
In the conventional CDMA communication system (IS-95.TM.) or wideband CDMA communication system, each of the base stations obtains the exact timing information from the GPS and then all the base stations operate in s synchronous mode. In this case, as shown in FIG. 1, each base station may be discriminated by the time offset of the same PN (Pseudo Noise) sequence. By doing so, 320 and 512 base stations may be discriminated with the time offset 62.5 .mu.sec and 52.1 .mu.sec, respectively.
For example, each base station of IS-95.TM., which has the PN sequence length 32768 and the chip rate 1.2288 Mcps, is discriminated with the time offset by a 64-chip (64.times.1/1.2288 M=52.1 .mu.sec), and each base station of "OKI" company, which has the PN sequence length 81920 and the chip rate 4.096 Mcps, is discriminated with the time offset by a 256-chip (256.times.1/4.096 M=62.5 .mu.sec).
As stated above, since it is possible to discriminate hundreds of base stations, the CDMA communication system has the advantage in that a designed can lighten the burden on the system design.
FIG. 2 is a schematic view illustrating a cell structure of the analogue cellular system and the time division multiple access (TDMA) system in the case of the reuse of seven frequency bands in accordance with the conventional communication system.
In the analogue cellular system and the TDMA system, only a few base stations may be discriminated because the base stations are discriminated by frequency. Accordingly, as shown in FIG. 2, some of the frequency bands must be used again. In this case, designers should lay a scheme capable of decreasing the interference between the base stations, using the same frequency. However, this scheme may impose a heavy burden on the designers because the propagation environment and the lay of the land have effects on the radio wave in communication.
For each base station to use time offsets different from each other, all the base stations must have the exact timing information between them. In particular, if the inaccuracy of the timing information is larger than the time offset, it is not possible to discriminate each of the base stations. Accordingly, in IS-95 and OKI, an exact synchronization between the base stations is obtained by the GPS. A communication system based on the GPS has many problems in its system stability and costs.
Therefore, a synchronous mode, which operates without the exact external timing information, is required between base stations. The present invention provides a method for using the timing information transmitted by a conventional wire or wireless network instead of the exact external timing information in the GPS.
The timing information transmitted by a conventional wire or wireless network is very inaccurate. Therefore, in the case where the base stations are discriminated using this timing information, the time offset between the base stations must be much larger than the inaccuracy of the timing information. However, this has a problem in that the time offset to discriminate the base stations decreases in number. On the other hand, in the case where the time offset increases in number, it requires lot of time in the initial synchronization.
Now, in the paging system, a synchronization between the base stations is set up using the timing information through a network because all the base stations must transmit signals at the same time. At this time, due to the distance between the base stations, time delay in network transmission lines and the processing time in modems transmitting and receiving signals, the inaccuracy of the synchronization between the base stations increases in time up to hundreds of microsends.
In the case where a plurality of base stations are discriminated by the time offset of the same PN code and the synchronization between the base stations is set up by a wire or wireless network, the base station is discriminated, in consideration of the time delay in the delay elements such as the transmission lines in each base station, by the time offset larger than the value having the largest inaccuracy. Also, since this value is not a fixed value, a large number of measurement data may be required. Furthermore, in the case where the system synchronization is abruptly unstable, the base stations may be not discriminated.
In the case where the exact timing information is not used, it is difficult to discriminate the base stations through the time offset of the same sequence. Accordingly, in Europe, the CODIT system), pursuing an asynchronous mode between the base stations, the discrimination between the base stations is achieved by allocating different sequences to each base station instead of the time offset of the same sequence. Although the base stations are not synchronized with each other, the base stations may be discriminated in the method of discriminating the base stations using the sequences different from each other.
However, in such cases, it takes the user a lot of time to acquire the code of a base station. In the case of IS-95 or OKI, since all base stations use the same sequence, the sequence of the base station may be acquired by performing the initial synchronization for only one sequence, but it is actually impossible to discriminate the base stations through the different sequences because the initial synchronization for all sequences must be performed.
A method to solve this problem is to set a limit to the number of sequences and to reuse the sequence as shown in FIG. 3 which is a schematic view illustrating a cell structure of the CDMA system in case of the reuse of seven frequency bands in accordance with the conventional communication system. However, this method has a problem in that it is impossible to easily lay a scheme of cells.
On the other hand, as stated above, the system, which obtains an exact synchronization using the GPS, has considerable problems in the system stability and costs.
Accordingly, a synchronous mode, which operates without the exact external timing information, is required between base stations.
The CODIT system in Europe or the NTT DoCoMo system in Japan makes researches in methods capable of being operated without exact synchronization information between the base stations, and not by using external timing information. To accomplish this purpose, only a method is to allocate different sequences to each base station, because the timing information is not known between the base stations, and then the base stations can not discriminate the time offset of the same sequence.
However, when the mobile station attempts an initial synchronization, the mobile station searches for only one sequence in the case where a mobile station uses the time offset of the same sequence, but in the case where the time offset is divided into different sequences, the mobile station searches for all the sequences. In the current digital cellular communication system, an initial synchronization is performed within a few seconds, but in the case where the mobile station is discriminated by the different sequences, the initial synchronization is performed for a long time. This means that the user waits for a few tens of seconds after dialing, it is impossible to apply this method to an actual communication service.
A method to solve this problem is also to set a limit to the number of sequences and to reuse the sequence as shown in FIG. 3. However, this method has a problem in that it is impossible to easily lay a scheme of cells, as stated above.